Di(sec-alkyl)-p-phenylenediamines are powerful anti-oxidants and antiozonants for the preservation of diene rubber. However, the lower members of the series are volatile skin irritants with the result that not only are they rapidly lost from the rubber with resultant loss of preservative action but cause serious discomfort to workmen. Higher members of 7 or 8 carbon atom alkyl groups having high antioxidant and antiozonant properties, acceptable low volatility and low skin burning effect are produced from ketones of 7 or 8 carbon atoms and have become widely accepted in the rubber industry. Unfortunately, ketones of 7 and 8 carbon atoms are limited in supply and expensive. Surprisingly, rubber preserved with dialkyl-p-phenylenediamine from the more available 6 carbon atom ketone, methyl isobutyl ketone, gives evidence of being a skin sensitizer. The ability of an agent to elicit a primary skin irritation is wholly different from that of producing skin sensitization. Moreover, the product from methyl isobutyl ketone is a mixture of stereo isomers which tends to form a semi-crystalline mass on standing.